<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 300px;
            height: 300px;
        }
    </style>
</head>
<body>
    <div></div>
    <script>
        function getRandomColor(flag=true){
            let mycolor
           if(flag){
            let arr = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f']
            mycolor='#'
            for(let i=1;i<7;i++){
                mycolor+=arr[Math.floor(Math.random()*15)]
            }
            return mycolor
           }
           else{
            mycolor=`rgb(${Math.floor(Math.random()*256)},${Math.floor(Math.random()*256)},${Math.floor(Math.random()*256)})`
            return mycolor
           }
        }
        console.log(getRandomColor())   //#ffffff  
        console.log(getRandomColor(true))   //#ffffff
        console.log(getRandomColor(false))   //rgb(255,255,255)
        const divv=document.querySelector('div')
        divv.style.backgroundColor=getRandomColor(false)
    </script>
</body>
</html>